1. Field of the Invention
The present invention relates to a nonwoven fabric containing fine fibers, and a filter material comprised of the nonwoven fabric. The present invention also relates to a fiber capable of generating fine fibers, the fine fibers generated therefrom, and a fiber sheet comprised of the fine fibers.
2. Description of the Related Art
A filter material acts to separate undesired solids, and a filter material comprised of a nonwoven fabric is widely used. The pore sizes of the filter material are preferably uniform, to ensure reliable consistency in filtration. Accordingly, the filter materials is preferably a nonwoven fabric prepared by a wet-laid method.
A nonwoven fabric prepared by forming a fiber web by a wet-laid method, and treating the fiber web with a water jet to entangle the web, is disclosed in, for example, Japanese Unexamined Patent Publications No. 2-6651, No. 3-14694, No. 4-222263, No. 4-240253, and No. 4-316653. The treatment with a water jet is carried out to impart strength to a nonwoven fabric. However, the water-jet treatment has a disadvantage in that a uniform texture of the fiber web is disturbed by the water-jet whereby a distribution of the pore sizes in the nonwoven fabric is made non-uniform and thus a desired filtering performance is lost.
Japanese Unexamined Patent Publications No. 63-232814 and No. 3-12208 disclose a nonwoven fabric which is prepared by a wet-laid method and contains fibrillated fibers. It is expected that the use of the fibrillated fibers brings about a bonding of fibers and thus enhances the denseness. However, the fibrillated fibers are liable to be entangled with each other, and therefore, it is difficult to disperse the fibrillated fibers in water as a dispersing medium and to prepare a nonwoven fabric having an excellent texture.
Further, when the fiber web is prepared by a wet-laid method, the fibrillated fibers are entangled with wires on which fibers are laid, and thus, when the laid web is peeled from the wire, the texture of the fiber web is deteriorated or a part of the fibers remains on the wires. Therefore, it is difficult to produce a nonwoven fabric having the desired properties.
Further, Japanese Unexamined Patent Publication No. 59-228918 discloses a wet-laid nonwoven fabric comprising 20 mass % or more of fine fibers having an average fiber diameter of 0.1 to 3 .mu.m, 20 mass % or more of intermediate fibers having a fiber diameter of 5 to 15 .mu.m, and 20 mass % or more of thick fibers having a fiber diameter of 20 to 50 .mu.m. In the nonwoven fabric, however, the presence of the thick fibers disturbs an orientation of the fibers, and large pores are formed in the vicinity of the thick fibers. Therefore, a distribution of the pore sizes becomes non-uniform and a desired filtering-performance is not obtained.
Further, it is believed that a separating performance of a filter material can be enhanced as an average diameter of fibers constituting the filter material becomes smaller. For example, a filter material composed of fine fibers having a diameter of about 5 .mu.m or less can effectively separate fine solids. Therefore, a diameter of the fibers for a filter material is preferably as fine as possible. The fine fiber preferably contains polypropylene because of a chemical resistance or an property.
A filter material comprising polypropylene fine fibers can be prepared, for example, by spinning islands-in-sea type fibers containing polypropylene island components, cutting the fibers into appropriate lengths, dissolving and removing the sea component, forming a fiber web from the island components, and bonding the fiber web. In the process as above, however, as the diameter of the polypropylene fibers becomes smaller, the island components are liable to bond with each other at cut surfaces due to a pressure applied when the islands-in-sea type fibers are cut. As a result, it is difficult to obtain a fiber web having a uniform texture and thus, to obtain a filter material having a uniform texture. Such an undesirable tendency is significant when the diameter of the island components is 2 .mu.m or less.